1. Field of the Invention
The present invention relates to a heat exchanger used for a cooling system such as an air conditioner, freezing/refrigerating apparatus and the like.
2. Description of Related Art
Recently, the ratio of heat pump type air conditioners using air as a heat source (hereinafter, referred to as a heat pump) to air conditioners has greatly increased and more than half of the room air conditioners used in homes and offices employ heat pumps. Further, most of the heat exchangers used in these heat pumps are a finned tube type heat exchanger composed of aluminum fins and coolant tubes perpendicular to the fins. In the heat pump, condensation forms on the surface of the fins of the heat exchanger disposed inside a room when air is cooled, and thus the amount of air passing through the heat exchanger is reduced by the bridge phenomenon caused by the water condensation between the fins, which results in the reduction of cooling capacity. Conversely, when air is heated, the same phenomenon as that of the above heat exchanger disposed inside the room arises in a heat exchanger disposed outside the room.
When the heat exchanger accumulates frost, the air flow resistance is increased and causes a reduction in cooling capacity. When the heat exchanger further accumulates frost, the fins become clogged due to the frost, which requires interruption of the heating operation and defrosting, and thus the comfort of heating is decreased. Consequently, to prevent the cooling and heating capacities from being reduced and the heat exchanger disposed outside the room from accumulating frost when air is heated and to reduce the number of defrosting operations to thereby improve comfort, the water condensed on the surface of the fins of heat exchangers of inside- and outside-room units must be removed at all times.
A method of removing the condensed water is to apply a water-repellent treatment to the surface of the fins to thereby cause the condensed water to fall down, and, for example, a method of coating with ethylene tetrafluoride resin, ethylene chlorotrifluoride resin and the like is known, as proposed by Japanese Utility Model Application Kokai (Laid-Open) No. Sho 51 (1976)-15261.
Since a heat exchanger to which this coating is applied has a contact angle of the surface of a fin with a water drop of about 110.degree., condensed water drops having a relatively large diameter of 2 mm or more can be caused to fall down from the surface of the fin.
The fin spacing of a today's heat exchanger, however, tends to be narrowed to increase the total surface area of the fins for the purpose of increasing the capacity of the heat exchanger. Today's heat exchangers generally have a fin spacing of about 2 to 3 mm and this spacing is expected to be further reduced hereinafter. When the fin spacing is reduced, a fine waterdrop having a diameter of about 1 mm will not drop from the surface of the fins by the method of coating with the above resin excellent in water repellency.
Accordingly, waterdrops remaining on the surface of the fins stay therebetween and act to retard air flow or are frozen to accumulate frost as it is, and thus the water-repellent effect of the method is not sufficient.